Conductive materials which are, to some degree, plastic are useful in various technological applications.
For example, conductive materials can be utilized as conductive filler in relatively lightweight shielding for electronic devices. This shielding is used to protect sensitive electronic devices from relatively high levels of electromagnetic radiation in the environment resulting from the increased utilization of electronic equipment.
More specifically, while protection can be provided by metallic shields, the substantial weight of solid metal shields, as well as their relatively high cost makes their use impractical. Moreover, housings for electronic components are often constructed in two or more parts to allow for ready access to the electronic components. Metallic shields are ineffective for sealing gaps between the parts of such housings, and therefore are not suited for protecting the components within from adverse atmospheric conditions. Accordingly, it is desirable to provide an EMI shield which is relatively lightweight and sufficiently plastic to act as a sealant, to protect housed electronic components from the elements, as well as to protect the components from EMI.
The basic requisite for such EMI shielding material is that it conduct electricity. Electrical conductivity can be imparted to plastics via incorporation of conductive fillers into the plastic matrix. This would seem in principle to be an acceptable solution. However, in practice there is a significant problem.
On the one hand, typical conductive fillers contain silver, nickel or copper. On the other hand, the housings for electronic components are typically made of aluminum. Because silver, nickel and copper are more noble than aluminum, these metals will set up a galvanic cell in contact with aluminum in the presence of moisture. In other words, there is an electrochemical potential difference between aluminum and the conductive fillers. This results in accelerated corrosion of the aluminum housing, which is referred to as galvanic corrosion (1). A filler which does not give rise to galvanic corrosion is needed.
Another application for conductive filler is incorporation in plastic material to provide an electrostatic charge dissipation composition. Such a composition can be deposited on, for instance, a metal surface so that, when a person carrying a static charge touches the coated surface, the charge is bled off by the conductive material in the composition, rather than discharged in a spark. Of course, the plasticity of the material is useful in conforming it to the surface's configuration, etc. But, as will be appreciated, the same galvanic corrosion difficulties as discussed above are attendant to use of electrostatic charge dissipation compositions containing conventional conductive fillers.
Ideally, to avoid galvanic corrosion, one could make the conductive filler from the same metal as that of which the housing is composed. Thus, in the case of an aluminum housing, aluminum powder would be used as a filler. However, the use of aluminum powder is disadvantageous in that the natural oxide film on the aluminum particles prevents the passage of electricity owing to the high resistivity of the oxide.
As a solution to the foregoing difficulty, the inventors identified in U.S. application Ser. No. 535,365 filed Jun. 8, 1990 now U.S. Pat. No. 5,175,056, have disclosed composite particles and compositions containing same. The particles comprise an electrically conductive core material and an electrically conductive refractory material. The core material, for instance, aluminum, has a surface oxide formation, but this does not prevent the use of the particle in utilities requiring conductivity (i.e., relatively low resistivity) because the refractory material is conductively fused to the core material and provides access to it through the oxide layer, typically breaching the layer. This overcomes the barrier to conductivity which would otherwise be posed by the oxide layer. Of course, because surface-oxide difficulties are obviated, the skilled practitioner is provided the freedom to choose a core material which is galvanically compatible with the housing or other metallic element that the particle-filled gasket or other material will abut.
This is a substantial progress over particles used heretofore, such as over aluminum particles, since long-term resistivity is appreciably lowered, and also over silver coated aluminum particles, since galvanic corrosion--characteristic with the latter--is suppressed. However, a further improvement in long-term resistivity would be a significant advance. Moreover, the initial resistivity of the composite particle mentioned above is incongruously (albeit only temporarily) high in comparison to that of other conventional particles. This has hampered the acceptance of that composite particle in industry. A decrease in initial resistivity would also be a great boon. Additionally, the aforementioned composite particle requires a relatively considerable amount of effort to disperse in binder matrix material when fashioning a gasket or other article of manufacture. A more dispersible particle would be highly advantageous.
Other galvanically compatible particles and compositions containing same are disclosed in PCT Application PCT/US91/04014 filed Jun. 7, 1991, published Dec. 11, 1991, under the number WO 91/18740. It is believed that an improvement of the resistivity and/or dispersibility of such particles can be improved even further, and that such improvement would be a desirable advance.